High Hardness and High Toughness Powder

ABSTRACT

In order to provide a projection material for shot peening, a raw material powder for a high-hardness powder metallurgy material having a superior hydrochloric acid resistance, a hard friction powder, a high hardness and high toughness alloy powder that can be used for hard particles for sintering and the like, the present invention provides a high hardness and high toughness alloy powder including in % by mass: a total of 25% or greater and 50% or less of one or two of Mo and W; 5% or greater and 15% or less of Cr; 0% or greater and 0.3% or less of Si; 0% or greater and 35% or less of Mn; 0% or greater and 20% or less of V; 0% or greater and 15% or less of Fe; and the balance including Co and inevitable impurities.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority based on the Japanese Patent Application No. 2015-249199, filed on Dec. 22, 2015, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a projection material for shot peening, a raw material powder for a high-hardness powder sintered compact having a superior hydrochloric acid resistance, a hard friction powder, a high hardness and high toughness alloy powder that can be used for hard particles for sintering and the like, as well as a projection material for shot peening using the same, a powder metallurgical composition, and a sintered compact.

Background Art

Conventionally, Co—Mo based and Co—W based alloys, as can be seen from each binary state diagram, produce various intermetallic compounds. These intermetallic compounds, having a high hardness, are suitable for various high hardness materials and wear-resistant materials. Also, Mo and W, by dissolving in Co, also have an effect of improving a corrosion resistance, inter alia, providing such a significant advantage that improves the corrosion resistance to reducing acids such as hydrochloric acid. Also, Co itself, as a base metal, has a high corrosion resistance against various acids. Therefore, powders of these alloy compositions can be used for a projection material for shot peening, a raw material powder for a high-hardness powder sintered compact having a superior hydrochloric acid resistance, a hard friction powder, hard particles for sintering, and the like.

For example, a CoMoCrSi-based alloy powder [Tribaloy®] disclosed in WO2012/063512A1 (Patent Document 1) has been extensively used. On the other hand, Si added for increasing the hardness of the alloy produces a rigid but brittle silicide, and reduces the bending strength of an alloy; thus posing a problem in applications requiring a particularly high bending strength. With those points in mind, for the purpose of improving hardness and corrosion resistance, even if Mo and W are to be added as much as possible, in light of the toughness such as bending strength, the additive amount should be actually kept low in this situation.

PRIOR ART DOCUMENTS Patent Document

Patent Document 1: WO2012/063512A1

SUMMARY OF THE INVENTION Technical Problem

An object of the present invention is to provide a projection material for shot peening, a raw material powder for a high-hardness powder sintered compact having a superior hydrochloric acid resistance, a hard friction powder, a high hardness and high toughness alloy powder that can be used for hard particles for sintering and the like, as well as a projection material for shot peening using the same, a powder metallurgical composition, and a sintered compact.

Solution to Problem

The present inventors closely examined the influence of the additive amount of Si on the Co-based alloy comprising a total of 25% by mass or greater of one or two of Mo and W; and found the scope that enables the alloy powder and the powder sintered compact to have a high toughness (i.e., the scope that enables the alloy powder to be difficult to crack, and enables the powder sintered compact to have a high bending strength); thereby completed the present invention. Furthermore, the present inventors found that, within the composition range of the alloy of the present invention, although a hard and brittle silicide has not been produced, by the formation of one or two Co—Mo based and Co—W based intermetallic compounds, age-hardening properties have been proven effective. For example, since a powder metallurgical composition comprising the alloy powder of the present invention is solidified by a powder molding/firing method such as HIP method (hot isostatic pressing method); and subsequently the hardness of a powder sintered compact can be changed by heat treatment, in order that the powder sintered compact is to be machined in a low hardness state; and then, the hardness the powder sintered compact can also be increased through an ageing treatment. Accordingly, the alloy powder of the present invention has made it possible to provide a powder sintered compact, which is machined in a low hardness state wherein it is easily machined, and which is available in a high hardness state wherein it provides an excellent wear resistance.

Consequently, the present invention encompasses the following alloy powder, projection material for shot peening, powder metallurgical composition, and sintered compact.

-   [1] An alloy powder, comprising in % by mass:     -   a total of 25% or greater and 50% or less of one or two of Mo         and W;     -   5% or greater and 15% or less of Cr;     -   0% or greater and 0.3% or less of Si;     -   0% or greater and 35% or less of Mn;     -   0% or greater and 20% or less of V;     -   0% or greater and 15% or less of Fe; and the balance consisting         of Co and inevitable impurities. -   [2] The alloy powder according to [1], wherein the alloy powder     comprises in % by mass, one or two or more selected from: greater     than 0% and 35% or less of Mn; greater than 0% and 20% or less of V;     and greater than 0% and 15% or less of Fe.     -   [3] The alloy powder according to [1] or [2], wherein the alloy         powder after ageing treatment has a Vickers hardness of 500 HV         or greater.     -   [4] The alloy powder according to any one of [1] to [3], wherein         the alloy powder has an age hardening width of 100 HV or         greater, wherein the age hardening width is defined by the         following formula: Vickers hardness of a sintered compact of the         alloy powder—Vickers hardness of the alloy powder before ageing         treatment.     -   [5] A projection material for shot peening, comprising the alloy         powder according to any one of [1] to [4].     -   [6] A powder metallurgical composition, comprising the alloy         powder according to any one of [1] to [4].     -   [7] A sintered compact obtained by sintering a molded article of         the powder metallurgical composition according to [6].     -   [8] The sintered compact according to [7], wherein the sintered         compact has a bending strength of 400 MPa or greater.

Advantageous Effects of Invention

As described above, the present invention provides a projection material for shot peening, a raw material powder for a high-hardness powder sintered compact having a superior hydrochloric acid resistance, a hard friction powder, a high hardness and high toughness powder that can be used for hard particles for sintering and the like, as well as a projection material for shot peening using the same, a powder metallurgical composition, and a sintered compact.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in more detail. It is to be noted that, in the present invention “%” means “% by mass”, unless otherwise specified.

Alloy Powder

The alloy powder of the present invention is characterized by comprising in % by mass: a total of 25% or greater and 50% or less of one or two of Mo and W; 5% or greater and 15% or less of Cr; 0% or greater and 0.3% or less of Si; 0% or greater and 35% or less of Mn; 0% or greater and 20% or less of V; 0% or greater and 15% or less of Fe; and the balance consisting of Co and inevitable impurities.

The greatest characteristic of the alloy powder of the present invention is that by reducing the additive amount of Si to less than that of a conventional CoMoCrSi alloy, both high hardness and high toughness are attained. Within the composition range of the alloy powder of the present invention, age-hardening properties are also exhibited. Further, as an additive element which does not affect these features, the use of one or two or more of Mn, V, and Fe is also possible.

It is to be noted that, available manufacturing methods for the alloy powder of the present invention include gas atomization, water atomization, disk atomization, pulverization of a rapidly cooled alloy foil strip or a casting material and the like, which have been known from the past. An alloy powder formed by a method such as a gas atomization method or a disk atomization method that provides a spherical shape (e.g., the lower limit of circularity by image analysis is between 0.85 and 0.75), and a method such as a water atomizing method that provides a generally spherical shape (e.g., the lower limit of circularity is between 0.80 and 0.70), when used as a shot peening projection material, is capable of suppressing the surface roughness of a projected material; or when used as a raw material powder for sintering, enables the filling rate at the time of molding to become high, which proves to be advantageous for forming a near net.

On the other hand, an alloy powder formed by a method such as various pulverization methods which provide an indefinite shape (e.g., the upper limit of circularity is between 0.80 and 0.70), when used as a projection material for shot peening as pretreatment for film formation processing such as thermal spraying, increases the surface roughness of a material to be projected, providing an effect of improving the adhesiveness of a film; or when used as a raw material powder for sintering, provides an effect of improving a shape retention property during molding.

The reasons why the component composition of the alloy powder of the present invention has been regulated will be described below. One or two of Mo and W: 25% or greater and 50% or less

In the alloy powder of the present invention, one or two of Mo and W are elements that increase the hardness of an alloy while decreasing toughness such as bending strength. When the total amount of one or two of Mo and W is less than 25%, a sufficient hardness may not be achieved. On the other hand, when the total amount of one or two of Mo and W is greater than 50%, toughness such as bending strength decreases. The total amount of one or two of Mo and W is preferably greater than 30% and less than 50%; and more preferably greater than 35% and less than 45%.

Cr: 5% or Greater and 15% or Less

In the alloy powder of the present invention, Cr is an element that, together with one or two of Mo and W, increases the hardness of an alloy, and also has an effect of improving a corrosion resistance, When the amount of Cr is less than 5%, the hardness and corrosion resistance will be insufficient, whereas when the amount of Cr exceeds 15%, the hardness before aging would become higher, thus leading to a reduced age hardening width. The amount of Cr is preferably 6 to 14%; and more preferably 7 to 13%.

Si: 0% or Greater and 0.3% or Less

In the alloy powder of the present invention, Si is an element that, by producing silicide, not only lowers the bending strength but also decreases the age hardening width, and thus the upper limit of the amount of Si is required to be specified. When the amount of Si exceeds 0.3%, the decrease of the bending strength has become marked, thus leading to a reduced age hardening width. The amount of Si is preferably 0.19% or less; and more preferably less than 0.15%. The lower-limit value of the amount of Si is not limited to a particular value, as long as it is 0% or greater. When Si is added, the lower-limit value of the amount of Si is not limited to a particular value, as long as it is greater than 0%; the lower-limit value of the amount of Si is, for example, 0.05%. Si may not be added (0.00%).

One or Two or More of Mn: 0% or Greater and 35% or Less, V: 0% or Greater and 20% or Less, and Fe: 0% or Greater and 15% or Less

In the alloy powder of the present invention, one or two or more of Mn, V and Fe are elements that will not impair the characteristics of the alloy powder of the present invention in the range not added excessively, and can be added as required. When Mn is added, the amount of Mn is adjustable within the range of greater than 0% and 35% or less as appropriate. Mn may not be added (0.00%). When V is added, the amount of V is adjustable within the range of greater than 0% and 20% or less as appropriate. V may not be added (0.00%). When Fe is added, the amount of Fe is adjustable within the range of greater than 0% and 15% or less as appropriate. Fe may not be added (0.00%). When the additive amount of each of Mn, V or Fe element exceeds 35%, 20% or 15% respectively, the bending strength decreases. On the other hand, Mn and Fe are elements that contribute to the reduction of raw material cost. Therefore, it is preferable to add Mn in excess of 20% and Fe in excess of 5%. Also, V is preferably less than 15% since it suppresses a decrease in bending strength.

The Vickers hardness of the alloy powder of the present invention after ageing treatment is preferably 500 HV or greater; and more preferably 700 HV or greater. The Vickers hardness of an alloy powder after ageing treatment is measured using a micro hardness tester “FM-700” from FUTURE-TECH for a test sample, which is prepared by removing coarse particles having a particle size exceeding 150 μm by means of fractionation using a sieve having an opening of 150 μm; then subjecting the obtained alloy powder having a particle size adjusted to 150 μm or less to ageing treatment in Ar at 800° C. for three hrs; after which, embedding the aged alloy powder in resin; and polishing the resultant. The test force in measurement of Vickers hardness is to be determined as 2.94 N (300 gf). Resins suitable for preparing a test sample include thermosetting resins. Other conditions conform to JIS Z 2244: 2009.

The age hardening width of the alloy powder of the present invention is preferably 100 HV or greater; and more preferably 200 HV or greater. The age hardening width of an alloy powder is defined by the following formula:

Vickers hardness of a sintered compact of the alloy powder—Vickers hardness of the alloy powder before ageing treatment.

The Vickers hardness of an alloy powder before aging treatment is measured using a micro hardness tester “FM-700” from FUTURE-TECH for a test sample, which is prepared by removing coarse particles having a particle size exceeding 150 μm by means of fractionation using a sieve having an opening of 150 μm; embedding the obtained alloy powder having a particle size adjusted to 150 μm or less in resin; and polishing the resultant. The test force in measurement of Vickers hardness is to be determined as 2.94 N (300 gf). Resins suitable for preparing a test sample include thermosetting resins. Other conditions conform to JIS Z 2244: 2009. The Vickers hardness of a sintered compact of an alloy powder is measured as follows: Coarse particles having a particle size exceeding 150 μm are removed, by means of fractionation using a sieve having an opening of 150 μm; the obtained alloy powder having a particle size adjusted to 150 pm or less (without ageing treatment) are loaded into in a stainless steel capsule having an inner diameter of 30 mm and a height of 30 mm, degassed, and encapsulated therein; to make a HIP molding at a holding temperature of 1150° C., for 3-h holding time, and at a molding pressure of 147 MPa; followed by slow cooling to form a sintered compact. The Vickers hardness of the sintered compact is measured in the same manner as the Vickers hardness of an alloy powder.

Projection Materialsf Shot Peening

The shot peening projection material of the present invention comprises the alloy powder of the present invention.

Powder Metallurgical Compositions

The powder metallurgical composition of the present invention comprises the alloy powder of the present invention.

The powder metallurgical composition of the present invention can be manufactured by mixing the alloy powder of the present invention and, if necessary, other powders (e.g., graphite powder).

By means of powder metallurgy using the powder metallurgical composition of the present invention, a sintered compact (a sintered alloy) can be manufactured. The alloy powder of the present invention included in the powder metallurgical composition of the present invention is useful as a raw material powder for a high-hardness powder sintered compact, a hard friction powder, hard particles for sintering, and the like.

The amount of the alloy powder of the present invention included in the powder metallurgical composition of the present invention, and other powders included as needed can be adjusted according to molding conditions, sintering conditions and the like used for powder metallurgy as appropriate.

Powder metallurgy using the powder metallurgical composition of the present invention can be carried out by a method comprising subjecting the powder metallurgical composition of the present invention to a compression molding to form a molded article (hereinafter referred to as “molding step”), and sintering said molded article to form a sintered compact (hereinafter referred to as “sintering step”).

A molding step can be carried out by, for example, loading the powder metallurgical composition of the present invention into a mold, which is then pressurized to form a powder compact. Before loading a powder metallurgical composition into the mold, a higher fatty acid lubricant may be applied to the inner surface of a mold. A molding step can be carried out using a known molding method such as pressing. Molding pressures and molding temperatures are to be adjusted as appropriate.

A sintering step can be carried out, for example, by heating and sintering a powder compact obtained by a molding step. A sintering temperatures and sintering time are to be adjusted as appropriate. Sintering atmosphere is preferably an oxidation preventing atmospheres such as a vacuum atmosphere, an inert gas atmosphere, a nitrogen atmosphere, and the like.

Powder molding/sintering methods include, for example, hot press method, hot hydraulic press method, powder extrusion method, powder forging method, and the like.

Sintered Compacts

The sintered compact of the present invention is obtained by sintering a molded article of the powder metallurgical composition of the present invention.

The bending strength of the sintered compact of the present invention is preferably 400 MPa or greater; and more preferably 800 MPa or greater.

The bending strength of a sintered compact is evaluated by performing a three-point bending test on a test piece having a length of 4 mm, a width of 25 mm and a thickness of 3 mm divided from a sintered compact using a wire. In the three point bending test, at an inter-fulcrum distance of 10 mm, a surface having a length of 4 mm and a width of 25 mm is pressed down, the stress (N) at that time is measured, and based on the following formula, the three-point bending strength is calculated.

Three-point bending strength (MPa)=(3×Stress (N)×Inter-fulcrum distance (mm)÷(2×Width of specimen (mm)×(Thickness of specimen (mm)²)

EXAMPLES

The present invention will now be described more specifically on the basis of examples.

First of all, the preparation of an alloy powder is to be described. Sample powder was prepared by a gas atomization method, a water atomization method, a rapidly cooled ribbon pulverization method or a casting pulverizing method.

In the atomization method, the dissolved raw material which weighed 25 kg was inductively dissolved in a refractory crucible under reduced-pressure Ar until 1750° C., tap water was discharged from a 7-mm diameter nozzle at the bottom of a crucible, atomization was performed using nitrogen gas or water as a spray medium.

In a quenching ribbon pulverization method, the dissolved raw material which weighed 30 g was subjected to an induction melting in a quartz tube under reduced-pressure Ar, tap water was discharged from a 1 mm nozzle at the bottom of a quartz tube into a copper roll having a diameter of 300 mm, a rotational speed of 500 rpm, to obtain a quenched ribbon, which was pulverized in a planetary ball mill substituted with Ar. The resultant was subjected to a solution treatment; i.e., it was vacuum-sealed in a quartz tube, held in a heating furnace at 1,200° C. for 1 hr, and then air-cooled.

In a casting pulverizing method, a dissolved raw material which weighed 200 g was subjected to arc melting under reduced-pressure Ar in a water-cooled copper mold having a diameter of 50 mm, a coagulated ingot was coarsely pulverized with a stamp mill, after which it was pulverized in a planetary ball mill substituted with Ar. The resultant was subjected to a solution treatment; i.e., it was vacuum-sealed in a quartz tube, held in a heating furnace at 1,200° C. for 1 hr, and then air-cooled.

It is to be noted that, the mean circularity, measured by means of PITA-1 from Seishin Enterprise Co., Ltd., is equal to or greater than 0.80 for a gas atomized powder; equal to or greater than 0.75 for a water atomized powder; and less than 0.75 for a pulverized powder.

With respect to the hardness of the alloy powder, coarse particles having a particle size exceeding 150 μm were removed, by means of fractionation using a sieve having an opening of 150 μm; the obtained powder having a particle size adjusted to 150 μm or less were embedded in resin; the resultant was then polished to prepare a test sample; and by measurement of the Vickers hardness of the test sample an evaluation was performed. Measurement of the Vickers hardness was carried out using a micro hardness tester “FM-700” from FUTURE-TECH, at a test load of 2.94 N (300 gf). For preparing the test sample, a thermosetting resin was used. The Vickers hardness of the test sample was evaluated by an average value of n =5. Also, the Vickers hardness was measured in the same way as a powder aged in Ar at 800° C. for 3 hrs.

With respect to the hardness and bending strength of a sintered compact prepared by powder metallurgy using an alloy powder, coarse particles having a particle size exceeding 150 μm were removed, by means of fractionation using a sieve having an opening of 150 μm; the obtained powder having a particle size adjusted to 150 μm or less (without ageing treatment) were is loaded into in a stainless steel capsule having an inner diameter of 30 mm and a height of 30 mm, degassed, and encapsulated therein; to make a HIP molding at a holding temperature of 1150° C., a holding time of 3 h, molding pressure of 147 MPa; followed by slow cooling and preparation of a sintered compact; and thus an evaluation was performed by measurement of the Vickers hardness of the sintered compact (the same method as in the case of powder), and the bending strength (the three point bending test with a fulcrum distance of 10 mm). The bending strength was evaluated by performing a three-point bending test on a test piece having a length of 4 mm, a width of 25 mm and a thickness of 3 mm divided from a sintered compact using a wire. The condition of three point bending test was performed at an inter-fulcrum distance of 10 mm; a surface having a length of 4 mm and a width of 25 mm was pressed down in the thickness direction; the stress (N) at that time was measured; and based on the following formula, the three-point bending strength was calculated. The calculated three-point bending strength was determined as the bending strength (MPa).

Three-point bending strength (MPa)=(3×Stress (N)×Inter-fulcrum distance (mm))÷(2×Width of specimen (mm)×(Thickness of specimen (mm)²)

For each evaluation item, a powder having a composition shown in Table 1 was prepared, an evaluation was performed, and thus the results shown in Table 1 were obtained. With respect to the hardness of powder, those having a Vickers hardness of a powder after the ageing treatment of equal to or greater than 700 HV were determined as “A”, those of less than 700 HV and equal to or greater than 500 HV were determined as “B”, and those of less than 500 HV were determined as “Cu”.

With respect to the age hardening width of a powder, those having a “Vickers hardness of a sintered compact of an alloy powder—Vickers hardness of an alloy powder before the ageing treatment” of equal to or greater than 200 HV were determined as “A”; those of less than 200 HV and equal to or greater than 100 HV were determined as “B”; and those of less than 100 HV were determined as “C”.

With respect to the bending strength of a powder sintered compact, those having a bending strength of the powder sintered compact of equal to or greater than 800 MPa were determined as “A”, those of less than 800 MPa and equal to or greater than 400 MPa were determined as “B”, and those of less than 400 MPa were determined as “C”.

TABLE 1 Age hardening Hardness width of Powder composition (% by mass) Manufacturing of sample sample Bending No Mo W Cr Si Mn V Fe Co Mo + W methods powder powder strength Notes  1 28 — 15 0.05 — — — Balance 28 CP B A A Inventive  2 28 — 15 0.05 25 — — Balance 28 CP B A B Examples  3 28 — 15 0.25  1 — — Balance 28 GA A B B  4 40 — 5 0.1 — — — Balance 40 RP A A A  5 40 — 5 0.3 — — — Balance 40 CP A B B  6 40 — 5 0.3 — 10 — Balance 40 CP A B B  7 18 18 10 0.15 — — — Balance 36 GA B A A  8 18 18 10 0.25 — — — Balance 36 WA A B A  9 18 18 10 0.25 — — 12 Balance 36 WA A B B 10 — 45 5 0.05 — — — Balance 45 WA B A A 11 — 45 5 0.2 — — — Balance 45 CP B A A 12 — 45 5 0.2 28 — — Balance 45 GA B A B 13 24 — 15 0.05 — — — Balance 24 CP C B A Comparative 14 — 52 5 0.2 — — — Balance 52 CP A B C Examples 15 — 45

0.2 — — — Balance 45 CP C A A 16 28 — 20 0.05 — — — Balance 28 CP A C A 17 28 — 15 2 — — — Balance 28 GA A C C 18 40 — 5 0.5 — — — Balance 40 CP A C C 19 18 18 10 1 — — — Balance 36 WA A C C 20 — 45 5 1.5 — — — Balance 45 CP B C C 21 28 — 15 0.05 37 — — Balance 28 CP B A C 22 40 — 5 0.3 — 24 — Balance 40 CP A B C 23 18 18 10 0.25 — — 18 Balance 36 WA A B C Note 1: Manufacturing methods (GA: Gas atomization, WA: Water atomization, RP: Rapidly cooled ribbon pulverization, CP: Casting pulverizing) Note 2: Underline items are excluded from the conditions of the present invention.

As shown in Table 1, No. 1 to 12 are Inventive Examples, while No. 13 to 23 are Comparative Examples.

The Comparative Example No. 13 shown in Table 1 has a lower total amount of a Mo element and a W element in a powder composition (<25% by mass), so that a Vickers hardness of a powder after the ageing treatment is regarded as a poorer one (evaluated as C). The Comparative Example No. 14 has a higher total amount of a Mo element and a W element in a powder composition (>50% by mass), so that the bending strength of a powder sintered compact was regarded as a poorer one (evaluated as C). The Comparative Example No. 15 does not contain Cr in a powder composition, so that a Vickers hardness of a powder after the ageing treatment and corrosion resistance was regarded as a poorer one (evaluated as C). The Comparative Example No. 16 has a higher Cr content in a powder composition (>15% by mass) so that the hardness of a powder before aging treatment becomes high, thus leading to a reduced age hardening width of a powder (evaluated as C).

The Comparative Examples No. 17 to 20 all have a high Si content in the powder composition (>0.3% by mass), so that the decrease of the bending strength of a powder sintered compact has become marked (evaluated as C), thus leading to a reduced age hardening width of a powder (evaluated as C). The Comparative Examples No. 21 to 23 all have a higher Mn, V and Fe contents in a powder composition (Mn: greater than 35% by mass, V: greater than 20% by mass, and Fe: greater than 15% by mass), so that the bending strength of a powder sintered compact was regarded as a poorer one (evaluated as C).

On the contrary, the Inventive Examples No. 1 to 12 all satisfy the conditions of the present invention, therefore proving the superiority of a Vickers hardness of a powder after the ageing treatment, the age hardening width of a powder, the bending strength of a powder sintered compact.

As has been described above, by comprising a total of 25 to 50% of one or two of Mo and W, a sufficient hardness is to be achieved; and at the same time, by comprising 5% to 15% of Cr, a corrosion resistance is to be improved; by limiting the upper limit of Si content to 0.3%, both a bending strength and an extensive age hardening width are to be attained, a projection material for shot peening having an excellent age hardenability and bending strength, a raw material powder for a high-hardness powder sintered compact having a superior hydrochloric acid resistance, a hard friction powder, a high hardness and high toughness powder that can be used for hard particles for sintering and the like can be provided. 

1. An alloy powder, comprising in % by mass: a total of 25% or greater and 50% or less of one or two of Mo and W; 5% or greater and 15% or less of Cr; 0% or greater and 0.3% or less of Si; 0% or greater and 35% or less of Mn; 0% or greater and 20% or less of V; 0% or greater and 15% or less of Fe; and the balance consisting of Co and inevitable impurities.
 2. The alloy powder according to claim 1, wherein the alloy powder comprises in % by mass, one or two or more selected from: greater than 0% and 35% or less of Mn; greater than 0% and 20% or less of V; and greater than 0% and 15% or less of Fe.
 3. The alloy powder according to claim 1, wherein the alloy powder after ageing treatment has a Vickers hardness of 500 HV or greater.
 4. The alloy powder according to claim 1, wherein the alloy powder has an age hardening width of 100 HV or greater, wherein the age hardening width is defined by the following formula: Vickers hardness of a sintered compact of the alloy powder—Vickers hardness of the alloy powder before ageing treatment.
 5. A projection material for shot peening, comprising the alloy powder according to claim
 1. 6. A powder metallurgical composition, comprising the alloy powder according to claim
 1. 7. A sintered compact obtained by sintering a molded article of the powder metallurgical composition according to claim
 6. 8. The sintered compact according to claim 7, wherein the sintered compact has a bending strength of 400 MPa or greater. 